A field chopper has at its front end a cutting unit that severs a standing crop, e.g. corn, at the base. The cut crop is moved back in the machine by conveyor rollers which compact it into a stream and feed it to a chopper which comminutes the cut crop and blows it out of the machine, typically via a tube or chute to a takeoff trailer or truck.
Hard objects that get past the front end cutter can pose a grave danger to the high-speed blades of the chopping drum. Accordingly commonly owned patent application Ser. Nos. 914,633 and 914,634 now U.S. Pat. No. 4,720,963 filed Oct. 2, 1986 describe systems for detecting such hard objects by reacting to the difference between the hard mass of such an object and the invariably softer crop.
Unfortunately metallic objects, normally pieces of wire or nails in pieces of wood, are occasionally found in or near a standing crop and are too small to be detected by these systems. Such objects can pose as great a danger to the chopping assembly as larger nonmetallic objects due to their considerable strength.
Accordingly it is known to provide electromagnetic detectors that can sense metallic objects, iron and steel ones being the principal problems. To detect such objects, German patent document No. 2,252,595 has permanent or electromagnets distributed over the width of the object stream so as to form a field whose lines of force run in the object transport direction, that is the direction the object stream is moving in. Two symmetrical coils are provided in this field which are offset relative to each other such that the blind spot any coil will inherently have in its center is covered by the other coil.
Such coils are quite expensive to manufacture, and two are absolutely necessary to scan the entire stream. Furthermore the device can occasionally be fooled when two objects symmetrically enter the field, something that happens often enough to be a problem. Thus this system, in addition to being expensive, is not capable of sensing all incoming objects.
German patent No. 2,430,147 describes a system wherein two separate magnets create separate but opposite fields whose lines of force extend parallel to the displacement direction of the stream through this field. The entire arrangement is surrounded by a coil. Such an arrangement also has a blind spot so that its application is also limited.
A system (See Agrartechnik international of April 1986) is known wherein the magnetic detecting system is mounted wholly within one of the feed rollers of the conveyor system. Once again the magnetic field produced extends in the direction of transport of the goods, being created by a north pole between two south poles. The north pole piece is provided with the detecting coil. This arrangement also can only detect magnetic objects that are relatively close, making use at the downstream end of the conveyor path where the stream is relatively thick impossible. The sensitivity also drops off so very much at the edges of the stream that there is virtually no detection here. Furthermore the long narrow sensing coil is particularly difficult and, therefore, expensive to manufacture.
In sum, none of the proposed solutions is highly sensitive across the entire cross section of the object stream being scanned for metallic objects. In addition all the known devices are expensive to manufacture and of normally relatively complex construction.